Quote:I have a slimline tray with fast flow waste - 90mm but the shower tray just fills and floods.
So what make/model of trap do you have?
Quite a few traps are designed to fit a big hole in the tray but the actual hole the water goes through is no bigger than a size standard trap.
If you need a high flow trap it would be reasonable to use 50mm/2" pipe but amazingly many big traps are designed for say 1.5" pipe and come with adaptors to suit 2" - so there is an inherent constriction compared to one that is 2" right through (if you get what I mean). Totally stupid design IMHO. Better traps are available.
The insert in most top access traps forms the u-bend/water seal so removing it or drilling holes in it would also reduce the depth of the u-bend. That could make it easier for sewer gas to get in. I suppose you could try that, if it causes a problem you could buy another identical trap and use the insert to reverse the change.
Simpler solution might be to use a lower flow shower head. It might even feel more powerful.